kselftest/alsa: pcm - Provide descriptions for the default tests

Help people understand what the standard tests are trying to cover by
providing descriptions which both serve as comments in the file and log
messages in the program's output.

Reviewed-by: Jaroslav Kysela <perex@perex.cz>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20221208-alsa-pcm-test-hacks-v4-6-5a152e65b1e1@kernel.org


Signed-off-by: Takashi Iwai <tiwai@suse.de>